Method for liquefying a hydrocarbon-rich flow

ABSTRACT

Disclosed is a method for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, by indirectly exchanging hear with the coolant mixture of a coolant mixture circuit. In said method, the coolant mixture is condensed in two or more stages, is divided into at least one lower-boiling and at least one higher-boiling coolant mixture fraction, and the coolant mixture fractions are evaporated at different temperature levels against the hydrocarbon-rich stream that is to be cooled and liquefied and are then combined before being condensed once again. The coolant mixture fractions are not entirely evaporated during normal operation and are preferably not superheated. Preferably, at least  1  to  10  percent by weight of the total amount of the coolant mixture are not evaporated.

The invention relates to a method for liquefying a hydrocarbon-richflow, in particular a natural gas flow, via indirect heat exchange withthe refrigerant mixture of a refrigerant mixture circuit.

A generic method for liquefying a hydrocarbon-rich flow is known forexample from German patent 197 22 490. By citing this document thedisclosure thereof shall be incorporated into the disclosure of thepresent patent application in its entirety.

Such methods for liquefying a hydrocarbon-rich flow, as described in theabovementioned patent, are generally designated as “Single-Flow MixedRefrigerant Cycle”. A common element here is that the refrigerantmixture is fully vaporised prior to (again) being fed to the circuitcompressor and is superheated by ca, 10 K above its dew point.

In the operating of generic methods for liquefying a hydrocarbon-richflow it has been shown however that this heating of the refrigerantmixture over and above the dew point can be highly problematic, sincethermal fluctuations arise in the region of the dew point, which cancause breakages in the pipes of lapped heat exchangers, preferably inthe outer pipe layers. The consequence of such pipe breakage isgenerally unwanted interruption of the liquefaction process.

Other generic methods for liquefying a hydrocarbon-rich flow are forexample the so-called “Parallel Mixed Refrigerant” method, as describedfor example in U.S. Pat. No. 6,389,844; the so-called “Double MixedRefrigerant” method, such as described for example in U.S. Pat. No.6,370,910; or other refrigerant mixture methods with pre-coolingcircuit, such as for example a “Propane-Mixed Refrigerant” method.

The aim of the present invention is to provide a generic method forliquefying a hydrocarbon-rich flow, by which the abovementionedconsiderable disadvantage can be avoided.

To solve this problem a generic method for liquefying a hydrocarbon-richflow is proposed, whereby the refrigerant mixture is compressedtwo-stage or multistage, is separated into at least one lower-boilingand at least one higher-boiling refrigerant mixture fraction and therefrigerant mixture fractions are vaporised at different temperaturelevels against the hydrocarbon-rich flow to be cooled and liquefied, andthen unified prior to recompression, and whereby the refrigerant mixturefractions are not fully vaporised.

A further advantageous accomplishment is that the refrigerant mixturefractions are not superheated, in particular they are not superheatedabove their dew point, and more particularly are not superheated by ca.10 K above their dew point.

In further advantageous configurations of the method according to theinvention for liquefying a hydrocarbon-rich flow, which constitute thesubject matter of the dependent claims, it is proposed thatindependently or in combination

-   -   at least 1 to 10% by weight, preferably at least 2 to 5 by        weight of the total quantity of the refrigerant mixture is not        vaporised;    -   incomplete vaporising of the refrigerant mixture is achieved by        the refrigerant mixture having a sufficiently large proportion        of heavier hydrocarbons, preferably C₄H₁₀— and C₅H₁₂—        hydrocarbons, depending on the selected method parameters, such        as vaporising temperature and pressure;    -   the liquid accruing from incomplete vaporising of the        refrigerant mixture fractions is separated off; and    -   this separated liquid is forwarded to a suitable supply point        inside the refrigerant mixture circuit, preferably by means of a        pump.

In contrast to the previously used methods from now on uninterruptedlyin normal operation there is no complete vaporising of the refrigerantmixture and its superheating above the dew point.

The quantity of liquid or fraction resp. resulting from incompletevaporising of the refrigerant mixture prior to feeding the refrigerantmixture to the circuit compressor is preferably separated off, since itcan otherwise result in damage to the circuit compressor. It is alsopossible to provide at least one extra pump, by means of which theseparated quantity of liquid or fraction resp. can be pumped to asuitable site inside the refrigerant mixture circuit. Referring forexample to FIG. 1 of the abovementioned German patent 197 22 490, thequantity of liquid or fraction resp. accruing in a separator D2 upstreamof the compressor V is fed to the separator D3, preferably using a pump.

Apart from the advantage of safe operation, which is associated with themethod according to the invention for liquefying a hydrocarbon-richflow, a not inconsiderable thermodynamic advantage can also be realised,Due to so-called “wet operation”, i.e. non-attainment of the dew pointinside the heat exchanger, the sharp knee at the dew point in theenthalpy temperature diagram of the refrigerant mixture is also avoided.The result of this is that the heating and cooling curves can be bettermatched, again resulting in an improvement in efficiency of the overallprocess.

The method can also be employed to advantage in other refrigerantmixture methods for liquefying hydrocarbon-rich flow, in addition to the“Single-Flow Mixed Refrigerant Cycle method”.

1. A method for liquefying a hydrocarbon-rich flow through indirect heatexchange with the refrigerant mixture of a refrigerant mixture circuit,whereby the refrigerant mixture is compressed two-stage or multistage,is separated into at least one lower-boiling and at least onehigher-boiling refrigerant mixture fraction and the refrigerant mixturefractions are vaporised at different temperature levels against thehydrocarbon-rich flow to be cooled and liquefied and are then unifiedprior to recompression, and whereby the refrigerant mixture fractionsare not fully vaporised.
 2. A method according to claim 1, whereby atleast 1 to 10% by weight of the total quantity of the refrigerantmixture is not vaporised.
 3. A method according to claim 1, wherebyincomplete vaporising of the refrigerant mixture is achieved by therefrigerant mixture having a sufficiently large proportion of heavierhydrocarbons depending on the selected method parameters.
 4. A methodaccording to claim 1, whereby the liquid accruing from incompletevaporising of the refrigerant mixture fractions is separated off.
 5. Amethod according to claim 4, whereby the separated resulting liquid isforwarded to a suitable supply point inside the refrigerant mixturecircuit.
 6. A method according to claim 5, whereby the forwarding iscarried out by means of a pump
 7. A method according to claim 1, wherebythe refrigerant mixture fractions are not superheated.
 8. A methodaccording to claim 2, whereby incomplete vaporising of the refrigerantmixture is achieved by the refrigerant mixture having a sufficientlylarge proportion of heavier hydrocarbons depending on the selectedmethod parameters.
 9. A method according to claim 1, whereby incompletevaporising of the refrigerant mixture is achieved by the refrigerantmixture having a sufficiently large proportion of heavier hydrocarbons,preferably C₄H₁₀— and C₅H₁₂— hydrocarbons, depending on the selectedmethod parameters, such as vaporising temperature and pressure.
 10. Amethod according to claim 2, whereby incomplete vaporising of therefrigerant mixture is achieved by the refrigerant mixture having asufficiently large proportion of heavier hydrocarbons, preferably C₄H₁₀—and C₅H₁₂— hydrocarbons, depending on the selected method parameters,such as vaporising temperature and pressure.
 11. A method according toclaim 2, whereby the liquid accruing from incomplete vaporising of therefrigerant mixture fractions is separated off.
 12. A method accordingto claim 3, whereby the liquid accruing from incomplete vaporising ofthe refrigerant mixture fractions is separated off.
 13. A methodaccording to claim 8, whereby the liquid accruing from incompletevaporising of the refrigerant mixture fractions is separated off.
 14. Amethod according to claim 9, whereby the liquid accruing from incompletevaporising of the refrigerant mixture fractions is separated off.
 15. Amethod according to claim 10, whereby the liquid accruing fromincomplete vaporising of the refrigerant mixture fractions is separatedoff.
 16. A method according to claim 11, whereby the separated resultingliquid is forwarded to a suitable supply point inside the refrigerantmixture circuit.
 17. A method according to claim 12, whereby theseparated resulting liquid is forwarded to a suitable supply pointinside the refrigerant mixture circuit.
 18. A method according to claim13, whereby the separated resulting liquid is forwarded to a suitablesupply point inside the refrigerant mixture circuit.
 19. A methodaccording to claim 14, whereby the separated resulting liquid isforwarded to a suitable supply point inside the refrigerant mixturecircuit.
 20. A method according to claim 15, whereby the separatedresulting liquid is forwarded to a suitable supply point inside therefrigerant mixture circuit.